Case study 2: Keeping down the Cost I am the manager of a frozen food storage de
ID: 345166 • Letter: C
Question
Case study 2: Keeping down the Cost I am the manager of a frozen food storage depot. One of my roles is to ensure the temperature in the storage area does not rise above -18 C since above that level the food would be deemed no longer safe to eat and have to be thrown away. However, my budget is to be cut and I have to find ways of managing the depot at lower cost. One option is to increase the proportion of refrigeration that takes place at night (between 23.30 and 06.30, the cost of electricity to power the refrigeration units is one third that of the day rate). At present, no attempt is made to maximise the proportion of night time refrigeration but I think I can do better by implementing a control policy, either by means of a simple time switch or by more sophisticated computer control (note, however, that the policy should be as simple as possible, consistent with achieving reasonable savings, and avoid frequent stop/start of units, which might damage them). There are three refrigeration units, each of which will operate down to temperatures of -23 C. Each is controlled by a thermostat which turn the units on (one for each unit). At present, one unit operates all the time. The thermostat for one of the other two units is currently set at just below -18'C and the other is currently set at exactly-18C. This is the maximum level to which the temperature can be allowed to rise. With only one unit on, the temperature rises. When it approaches -18°C the second unit is turned on. If the temperature continues to rise, it reaches-18C and the third unit is turned on. Refrigeration then continues until the temperature falls to -23'C, at which point the second and third units are turned off. Periodically, the order of the units is changed to prevent excessive wear to one of them The rate at which the temperature rises varies. In the coldest winter months, the temperature will rise at 1.2C an hour with all the units off; in the hottest summer months, it will rise at 1.8C an hour. Each unit will reduce the temperature by 0.8°C an hour, and costs £120 per hour to run at the day time rate What sort of control policy do you recommend? How much will this save me? I might be able to obtain a fourth unit on the cheap and wonder if it would be worth installing. Would installing a fourth unit enable me to save significantly more on electricity costs? I could also modify the units to lower the minimum temperature at which they will operate, perhaps by 1.0°C, increasing the time the temperature takes to rise to -18'C. Would this make a differen Or I could do both, if it would helpExplanation / Answer
Upon reviewing the case the best control policy is to modify the units at which they operate. The rate can be increased by 1.0’C. This will enable the cost to reduce and also if there is a space to install the 4th unit it would also help since there will be a continuous flow of the electricity and does not let the temperature to increase. The 4th unit can be used in the night time since the electricity rates are less when compared to day time.
So, both the Options are beneficial. This could save up to 9.6 pounds.